be a dab hand at something/at doing something

be a dab hand at

be expert at.
Dab in this sense is recorded since the late 17th century, but its origin is unknown.
1998 Bookseller Stephanie Cabot…is apparently a dab hand at milking cows, according to one of those mystifying diary items in Skateboarders' Weekly.
See also: dab, hand

be a dab ˈhand at something/at doing something

(British English, informal) be very good at doing something: Ask Neil to do it — he’s a dab hand at carpentry.
See also: dab, hand, something