The phrase 'Handsome man' literally comes from the Thai phrase "ผู้ชายหล่อ" (phoochai-lo, ผู้ชาย phoo-chai = man, หล่อ lor = handsome, good-looking). It is commonly used in the Thai language to describe a good-looking or handsome man.
Currently, this is casual slang, so Thais will use "ผู้ชายหล่อ" casually when referring to any good-looking guy, be it a friend, acquaintance, or stranger on the street. This usage is also influenced by pop culture and celebrity trends, as Thai media is filled with good-looking actors, singers, and models. Describing male celebrities as "ผู้ชายหล่อ" is very prevalent in entertainment news and gossip.
So when you hear 'handsome man,' it's a mirror translation from Thai. However, I would take this compliment with a grain of salt, as our handsomeness is often considered in proportion to the thickness of our wallet.