(British smoking slang) When you're chilling back with your homeboys smoking up a phat joint, sometimes one will note that one side of the "bifta" burns faster than the other, and will sometimes curl upwards. When this occurs, the joint is said to be a U-boat. This usually happens because of a poorly rolled, uneven joint, or because of excess saliva on the gum side if it has just been rolled. Some synonyms for "U-boating" are "elvising" (in reference to the famous quiffed hairstyle)and "sideying" ( as it is only burnt on one side.)
"What wanker rolled this fucking joint? It's u-boating like a bitch!"
"That was a total waste of draw, we only smoked half of it 'cos it sideyed!"