also: ふたえあご
noun
double chin
Refers to the fatty or loose skin under the chin that creates a second chin-like appearance.
最近、二重顎が気になってきた。
Lately, I've become self-conscious about my double chin.
二重顎を解消するエクササイズを始めた。
I started exercises to get rid of my double chin.
顎 means 'chin' or 'jaw' in general, while 二重顎 specifically refers to a double chin.
Compound of 二重 (にじゅう, 'double') and 顎 (あご, 'chin'). The alternate reading ふたえあご uses the native Japanese reading ふたえ for 二重.