Mister Woof was a name that Ambassador Lwaxana Troi occasionally called Worf by mistake.
Following, Worf's protest to Troi's "unauthorized presence" on the USS Enterprise-D's bridge in 2367, she indicated to a position on his tactical console, asking, "What does that little one do, Mister Woof?" He frustratedly replied, "Please, Madame! That is a torpedo launch initiator, and, it is Worf, Madame, not Woof." (TNG: "Half a Life")
Despite Worf correcting her, she continued to use the name, as she did in 2368, when Ambassador Troi went on about "what a wonderful name" his son Alexander had, before learning from Deanna Troi that Worf and Alexander had a contract worked out between them in order to change Alexander's conduct, she asked, "and you're doing this to your own child, Mister Woof?" Like their previous exchange, Worf explained to the ambassador, "It is Worf, Madam." (TNG: "Cost Of Living")
On one final occasion in 2370, during the Cairn visit to the Enterprise-D, Troi asked, "aren't you going to mingle, Mister Woof?" Worf did not correct her this on occasion, but instead referred to the fact that he "[did] not care for telepaths. They make me uneasy." (TNG: "Dark Page")