tushy (n.)

"backside, buttocks," 1962, an abbreviation of tochus (1914), from Yiddish tokhes, from Hebrew tahat "beneath."

suffix in pet proper names (such as Johnny, Kitty), first recorded in Scottish c. 1400; according to OED it became frequent in English 15c.-16c. Extension to surnames seems to date from c. 1940. Use with common nouns seems to have begun in Scottish with laddie (1546) and become popular in English due to Burns' poems, but the same formation appears to be represented much earlier in baby and puppy.