덩달아 + 웃다/기뻐하다/따라 하다
following others, along with others, likewise
Example
친구가 웃자 나도 덩달아 웃었어요.
When my friend laughed, I laughed along too.
Word parts
From 덩달다 'to follow along without one's own reason'.
Good to know
Implies you joined in because others did, not for your own reasons.
Related words