What did "on by out", "on by up", "on by over" mean?
Why did Old English tack and jam these different prepositions together? E.g. didn't ufan alone mean "above"? Why prefix it with a- and -b- that appear to conribute nothing to the meaning?
About in Old English times meant ‘around the outside of’; it did not develop its commonest present-day meaning, ‘concerning’, until the 13th century. In its earliest incarnation it was onbūtan, a compound made up of on and būtan ‘outside’ (this is the same word as modern English but, which was itself originally a compound, formed from the ancestors of by and out – so broken down into its ultimate constituents, about is on by out).
→ BUT, BY, OUT
As in the case of about, the a- in above represents on and the -b- element represents by; above (Old English abufan) is a compound based on Old English ufan. This meant both ‘on top’ and ‘down from above’; it is related to over, and is probably descended from a hypothetical West Germanic ancestor *ufana, whose uf- element eventually became modern English up. So in a sense, above means ‘on by up’ or ‘on by over’.
→ BY, ON, UP
Word Origins (2005 2e) by John Ayto. p 2.