In polite circles, "Trasero" is seldom used for refering to the butt but rather 'the lower back'.
Pendejo: coward, Idiot, and would only be used as Prolijo suggested in the case of two very good friends, and then only when both parties knew for certain it was in jest!!
Culo is the word most commonly used, often even in polite conversations, to refer to the butt or ass.
Cola- As stated is the "tail" and is concidered very vulgar and is never used in mixed company except by some gringos who do not know better (like me a few years ago with my ex wife-damn did I find out what a temper that Colombiana had)
Regarse: Another word more commonly used with the same meaning when used in the same context is "polvo"
And a couple of other commonly use words:
Maes: friends
chante; building, house, etc. Commonly used to refer to a Sala de Masaje (MP)
Compas: shortened form of companarios (companions)
One that I have always found interresting is the verb 'coger' to take or catch. like 'to take the bus'. Problem is in many latin countries it is used to refer to 'Plucking" and if you say "cogere el autobus' ( "I will take the bus") you are saying "I will pluck the bus".
Just as in english, there are many, many clolqual words and phrases that have very different meanings in different regions or countries, which makes using the right word or phrase very difficult at time for the non local, non native speaker.
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Old and retired but still bang, and bang, and bang!!!
