One who does not abide by rules.
Etymology: from the surname of Texas lawyer Samuel Maverick, who refused to brand his cattle.
A self-styled expert in a given field.
Etymology: from Yiddish מבֿין (meyvn) from Hebrew מֵבִין (mevín, “one who understands, connoisseur, expert”).
Everyone loves a maverick, but mavens get it done.