ケツ, ケン, か.ける, か.く, あくび
lack, gap, fail, yawning radical (no. 76)
JLPT3 G4 S4 F860
A depiction of a bent, open-mouthed figure (with food contained in the mouth) → *yawn* (← open mouth). 欠 now also bears the senses of 缺 (hole scooped into earthenware to create a container), accounting for the meanings *scoop out* → *lack*; *miss/be missing*; *break* (off); *chip* (off); *broken piece*; *fragment* → *flaw*.
[Full Kanji Etymology List]