Chin

From Teletraan I: The Transformers Wiki

Long emotional abuse over his chin may have made Sentinel the jerk he is today. Or he was just built that way.

A chin is the lowest part of the jaw on creatures with mouths. Transformers have had chins since as long as they've had faces. Why is something of a mystery, since Transformers do not technically need mouths (and some don't have one). One must also ask why certain bots have such large chins when they don't actually need them. Humanization? Experiences with mammalian lifeforms? Required for socially acceptable stroking when deep in thought? Only Primus knows the truth.