From 1d4chan

Healslut is a somewhat derogatory term used to refer to clerics, especially to ones where the player character and/or the player themselves is female. The term comes from the common perception of the healing-focused cleric as a "girly" class, in that female players are (were?) perceived to be more likely to prefer the class that heals instead of kills, and consequent jokes about the erotic aspects of healing when administered by a sexy woman.