godan verb (-mu), intransitive verb
chuckle to oneself; gloat
Describes a quiet, self-satisfied laugh, often with a negative connotation of smugness or schadenfreude.
彼は失敗した同僚を見てほくそ笑んだ。
He chuckled to himself as he watched his colleague fail.
ライバルのミスにほくそ笑むのは
It's not good to gloat over a rival's mistake.