The unexpurgated version is as is by Burns. If you look at the words carefully and with an open mind the following appears to be the case.

The whole song is about a sexual encounter in a field behind the house at Cherry Trees, which does or did exist. 'I trow he lay na lang at that,   But turned and in a verra crack, produced a 'Dainty Davie' I think it is obvious what he produced and later put in her hand. Atweesh ma thees' translated would be between my thighs, 'Splash! gayed oot his gravy', well that does not need translating, I don't think.

Contributors have tried to explain that a curly pow is hair, well I think it is but not of the head variety, more likely it is the hair which surrounds the 'Dainty Davey' which was put in her hand and from whence the gravy splashed. Burns was well known for writing suggestive if not blatantly disgusting words. Nine inches will please a Lady, for one