a person who makes plans and/or says yes to anything when drunk, then at the next sober point in their life, changes their mind and/or retracts aforementioned plans or agreements
That drunken yesser, Bob, said last night, that he was going camping with us. Today he said he can't afford it.
Me to friend 'Can we come stay Saturday?' friend 'Absolutely. Looking forward to it already', Me 'Yaysers!' Friend 'New word of the day! Like it a lot! Xxx'