large face, flat face, stupid
頇 centers on a large, flat face, often with a connotation of dullness or stupidity. The meanings are closely tied to facial appearance and the associated impression.
頇 combines 頁 (head) with 干, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with a large or flat face.
The left side 干 looks like a flat, wide surface, and the right side 頁 means head or face. Imagine a face as flat and broad as a shield (干), giving a dull expression.
For カン, picture a large, flat face that looks like a can: can -> カン, and the face is as wide and expressionless as a tin can.