According to Alexander Beider´s dictornary of surnames of the kingdom of Poland, Lak (and his derivation Lok)  means sealing wax and, therefore, an occupational surname that was common in Biala, Wegrów, Zamosc and Warsaw.
If Lak is written with a slash on the L then it is excatly like my mother´s maiden name Lach which means Pole, the genitive form of the people of Poland.

Alejandro T. Rubinstein Lach