The story
Blend of moo (“lowing sound of a cow”) + woof (“barking sound of a dog”). Coined by American computer programmer Mark Harlan in 1987 or 1988 in internal memoranda within Apple Computer. First publicly attested in 1989. Trademarked by Apple from 1992 to 1999 (U.S. Reg. No. 1,695,826).
Etymology from Wiktionary, CC BY-SA 4.0.