This would be a case of jukujikun I believe.
I’m just pulling this from Wikipedia.
Kanji - Wikipedia
Most often, a character will be used for both sound and meaning, and it is simply a matter of choosing the correct reading based on which word it represents. In other cases, a character is used only for sound (ateji). In this case, pronunciation is still based on a standard reading, or used only for meaning (broadly a form of ateji, narrowly jukujikun). Therefore, only the full compound—not the individual character—has a reading. There are also special cases where the reading is completely different, often based on an historical or traditional reading.
Special readings[edit]
Gikun (義訓?) and jukujikun (熟字訓?) are readings of kanji combinations that have no direct correspondence to the characters’ individual on’yomi or kun’yomi,. From the point of view of the character, rather than the word, this is known as a nankun (難訓?, difficult reading), and these are listed in kanji dictionaries under the entry for the character.
Gikun are when non-standard kanji are used, generally for effect, such as using 寒 with reading fuyu (“winter”), rather than the standard character 冬.
Jukujikun are when the standard kanji for a word are related to the meaning, but not the sound. The word is pronounced as a whole, not corresponding to sounds of individual kanji. For example, 今朝 (“this morning”) is jukujikun, and read neither as *ima’asa, the kun’yomi of the characters, nor konchō, the on’yomi of the characters, nor any combination thereof. Instead it is read as kesa, a native bisyllabic Japanese word that may be seen as a single morpheme, or as a fusion of kyō (previously kefu), “today”, and asa, “morning”. Likewise, 明日 (“tomorrow”) is jukujikun, and read neither as akari(no)hi, the kun’yomi of the characters, nor meinichi, the on’yomi of the characters, nor any combination thereof. Instead it is read as ashita, a native bisyllabic Japanese word that may be seen as a single morpheme. Jukujikun are primarily used for some native Japanese words, such as Yamato (大和 or 倭, the name of a Japanese province as well as ancient name for Japan), and for some old borrowings, such as shishamo (柳葉魚?, willow leaf fish) from Ainu, tabako (煙草?, smoke grass) from Portuguese, or bīru (麦酒, wheat alcohol) from Dutch. Words whose kanji are jukujikun are often usually written as hiragana (if native), or katakana (if borrowed); some old borrowed words are also written as hiragana, especially Portuguese loanwords such as karuta (かるた) from Portuguese “carta” (Eng: card), tempura (てんぷら) from Portuguese “tempora”, and pan (ぱん) from Spanish “pan” (Eng: bread), as well as tabako (たばこ).
Jukujikun are quite varied. Often the kanji compound for jukujikun is idiosyncratic and created for the word, with the corresponding Chinese word not existing; in other cases a kanji compound for an existing Chinese word is reused, where the Chinese word and on’yomi may or may not be used in Japanese; for example, (馴鹿?, reindeer) is jukujikun for tonakai, from Ainu, but the on’yomi reading of junroku is also used. In some cases Japanese coinages have subsequently been borrowed back into Chinese, such as ankō (鮟鱇?, monkfish).
The underlying word for jukujikun is a native Japanese word or foreign borrowing, which either does not have an existing kanji spelling (either kun’yomi or ateji) or for which a new kanji spelling is produced. Most often the word is a noun, which may be a simple noun (not a compound or derived from a verb), or may be a verb form or a fusional pronunciation; for example sumō (相撲?, sumo) is originally from the verb suma-u (争う?, to vie), while kyō (今日?, today) is fusional. In rare cases jukujikun is also applied to inflectional words (verbs and adjectives), in which case there is frequently a corresponding Chinese word.
Examples of jukujikun for inflectional words follow. The most common example of a jukujikun adjective is kawai-i (可愛い?, cute), originally kawayu-i; the word (可愛?) is used in Chinese, but the corresponding on’yomi is not used in Japanese. By contrast, “appropriate” can be either fusawa-shii (相応しい?, in jukujikun) or sōō (相応?, in on’yomi) are both used; the -shii ending is because these were formerly a different class of adjectives. A common example of a verb with jukujikun is haya-ru (流行る?, to spread, to be in vogue), corresponding to on’yomi ryūkō (流行?). A sample jukujikun deverbal (noun derived from a verb form) is yusuri (強請?, extortion), from yusu-ru (強請る?, to extort), spelling from kyōsei (強請?, extortion). See 義訓 and 熟字訓 for many more examples. Note that there are also compound verbs and, less commonly, compound adjectives, and while these may have multiple kanji without intervening characters, they are read using usual kun’yomi; examples include omo-shiro-i (面白い?, interesting) face-whitening and zuru-gashiko-i (狡賢い?, sly).
Typographically, the furigana for jukujikun are often written so they are centered across the entire word, or for inflectional words over the entire root – corresponding to the reading being related to the entire word – rather than each part of the word being centered over its corresponding character, as is often done for the usual phono-semantic readings.
Broadly speaking, jukujikun can be considered a form of ateji, though in narrow usage “ateji” refers specifically to using characters for sound and not meaning (sound-spelling), rather than meaning and not sound (meaning-spelling), as in jukujikun.
Many jukujikun (established meaning-spellings) began life as gikun (improvised meaning-spellings). Occasionally a single word will have many such kanji spellings; an extreme example is hototogisu (lesser cuckoo), which may be spelt in a great many ways, including 杜鵑, 時鳥, 子規, 不如帰, 霍公鳥, 蜀魂, 沓手鳥, 杜宇, 田鵑, 沓直鳥, and 郭公 – many of these variant spellings are particular to haiku poems.
