E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Putting on Side.
Giving oneself airs. Side is an archaic word for a train or trailing gown; also long, as his beard was side. A side-coat means a long trailing coat. (Anglo-Saxon sid, great, wide, longas sid-feax, long hair.)
I do not like side frocks for little girls.Skinner.