noun
side job; sideline
Work done in addition to one's main occupation, often implying it is manageable with one hand (i.e., not a full-time commitment).
彼は本業のほかに片手業で翻訳をしている。
He does translation as a side job in addition to his main work.
noun
one-handed act; doing something with one hand
Literally an action performed using only one hand; can also imply doing something easily or casually.
It's dangerous to operate a smartphone with one hand.