Say but not really mean

MOUTH

["This body part can speak words that aren't always sincere."]

The clue "Say but not really mean" hints at the word "mouth," which can mean to speak or articulate words ("say") but also refers to the physical mouth that doesn't actually "mean" anything itself. It plays on the dual meaning of "mouth" as both a verb (to speak) and a noun (the body part). The wordplay suggests saying something without genuine intent, tying the two definitions together.