Best definition
A term of endearment. It is the name given to a sibling-type friend for whom one loves dearly. Wallykins has

a sensational talent with regards to music esp. with the guitar. He has the voice of a thousand arch angels which resonates like a cacophony of bells in the great basilica of the heavens. (wal, walla, wally, wallypoppykins, wallypoppysloppyploppykins)

When blushing, modestly, from a complement, one shall reply….. “Love you too wallykins”