c. 1400, hobi, "small, active horse," short for hobyn (mid-14c.; late 13c. in Anglo-Latin), probably originally a proper name for a horse (compare dobbin), a diminutive of Robert or Robin. Old French hobi, hobin, once considered possible sources, now are held to be borrowings from English.
The modern sense of "a favorite pursuit, object, or topic" is from 1816, a shortening of hobbyhorse (q.v.) in this sense, which is attested from 1670s. Earlier it meant "a wooden or wickerwork figure of a horse," as a child's toy or a costume in the morris-dance, the connecting notion being "activity that doesn't go anywhere." Hobby as a shortening of hobbyhorse also was used in the "morris horse" sense (1760) and the "child's toy horse" sense (1680s).
word-forming element meaning "one who does or makes," also used to indicate adherence to a certain doctrine or custom, from French -iste and directly from Latin -ista (source also of Spanish, Portuguese, Italian -ista), from Greek agent-noun ending -istes, which is from -is-, ending of the stem of verbs in -izein, + agential suffix -tes.
Variant -ister (as in chorister, barrister) is from Old French -istre, on false analogy of ministre. Variant -ista is from Spanish, popularized in American English 1970s by names of Latin-American revolutionary movements.
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Definitions of hobbyist from WordNet
a person who pursues an activity in their spare time for pleasure;