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1.
(1) The hidden underlying reference to squareness, and boat-like shape, of the heads on Singaporeans (originally designed by the Merry Boat Heads clan as a way to insult without having to face the penalties/risks of being "offensive").

(2) It also can be referenced to the greatest gaming clan in Singapore - the notorious Merry Boat Heads.

Being insulted as a boathead is probably the worst thing you can imagine - yet each member of the Merry Boat Heads is, ironically, able to be compared to that of a God; they are each that amazing.

(3) It's a professional troll word - used by only the masters.
You're a boathead!
Don't worry; The Merry Boat-Heads crushed you - it's expected since the start!
Boathead!
by WCGPR0 February 27, 2012
 
2.
Giving a man oral sex on his watercraft.
Maria gave boat head to Tyler in the speed boat.
by Lindsay lowone February 17, 2009
 
3.
The act of an individual giving oral sex to a man while he drives his boat gracefully through the open sea. This is especially good because the waves help the person performing the oral sex maintain a steady up and down motion without even moving.
Kristin gave her boyfriend Mike boat head on a nice sunny afternoon
by TRN123 June 04, 2014