A person who’s properly good at something, like they’ve done it a thousand times and could do it half asleep. You’ll hear it as “a dab hand at…” followed by the skill, from cooking to fixing cars. It’s not flashy genius, it’s calm, practised competence. Honestly, top-tier compliment for the quietly capable.
"Mum’s a dab hand at DIY, mate. Boiler’s coughing like a smoker, she gives it a whack with a spanner and goes, sorted. Meanwhile I can’t even change a bulb."