Casual, masculine-leaning apology used among friends or in informal situations. Less formal than すみません.
See also: すみません
すまん、遅くなった。
Sorry I'm late.
すまん、ちょっと通してくれ。
Excuse me, let me through.
すまん is a casual, masculine-leaning contraction used with friends, while すみません is the polite, standard form used with strangers or superiors.
From 済まぬ, the classical negative form of 済む in the apologetic expression meaning that an offense or obligation “is not settled.” The ending later contracted from ぬ to ん, producing 済まん.