When someone is strapping, they're strikingly strong looking. Why don't you ask those strapping friends of yours to help you carry that piano up the stairs?
The adjective strapping usually describe someone who appears to be young, muscular, and robust. When it was first used in the 17th century, it almost always referred to a tall, sturdy woman. Today, it's more often used for young men. Strapping is part of an odd category of adjectives meaning "large" which are formed from violent verbs — to strap is to "beat with a strap." Others are whopping and spanking.