吻 is more literary and romantic than 亲 (qīn), which is the everyday colloquial word for 'to kiss.' You'd see 吻 in novels and movies, while 亲一下 is what people actually say in daily life.
Left mouth-radical — the indexing radical. The mouth is what does the kissing — and the original sense was 'corner of the mouth, lips,' before generalising to the act of kissing itself. Anchors 吻 in the body-action-with-mouth family alongside 唇 (lip), 喉 (throat).
Right 勿 supplies the sound (wù → wěn, an irregular but established alternation). Same phonetic appears in 物 wù (thing), 刎 wěn (cut throat). Semantically empty here — pure sound carrier. Mnemonic hook: don't do things lightly with the lips.
