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).
He does translation as a side job in addition to his main work.
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.
Compound of 片手 (one hand) and 業 (work, act). The exact historical derivation is uncertain, but the word literally suggests work that can be done with one hand, hence a side job or a one-handed action.