Be no slouch
Definition 1 of 1
Pro Tip 1/3Positive Connotation
The phrase implies respect for someone's ability or skill in a specific area. 
Despite his modesty, he's no slouch when it comes to public speaking.
Pro Tip 2/3Informal Usage
It's used in informal contexts, such as conversations with friends, and not in formal writing. 
I met the new teacher; she's no slouch in managing the classroom.
Pro Tip 3/3Contextual Clarity
Make clear what skill or activity is being referred to, to avid misunderstandings. 
In the kitchen, Maria is no slouch—her desserts are famous.