A wemmel is best described as something with a wobble on it, and can be used instead of the word wobble. For example a chair can have a "wemmel" on it.
wemmel can also be used in a plural form, wems, which has a different meaning. A girl can usually be found with a cracking pair of wems, but wems should only be used when the pair of breasts in discussion are in fact cracking.
"Christ, we'll have to fix that table, it's got a hell'uva wemmel on it."
"Christ, did you just see Faye? She has a cracking pair of wems."