What's the origin of 'Butter her up', i.e. to make her more amenable to my intentions?
I think origin is plainly related to sex. We never use this term in relation to men, do we? I also think that this might be the origin of the related term 'Butter wouldn't melt in her mouth!', which may refer to the question of where would it melt, then?; implying that this female is so innocent that butter wouldn't melt anywhere on her body - not even in her mouth!
Maybe this was / is a term used by ordinary people. The vulgar sexual meaning might be the reason that no author ever has - to my knowledge - clearly explained it.
This seems, to me, to be the most obvious meaning and origin. Any counter-suggestions or comments?