godan verb (-su)
clench (teeth); bite together
Primarily used for clenching the teeth, often in anger or determination. The kanji 食い合わす suggests biting together. This verb is somewhat rare and may be replaced by 食いしばる in modern usage.
彼は悔しさに歯を食い合わせた。
He clenched his teeth in frustration.
More common modern verb for clenching teeth, especially in enduring pain or effort.
Compound of 食い (kui, 'eating' or 'biting') and 合わす (awasu, 'to join together'). The exact historical derivation is uncertain, but the meaning is transparent from its components.