Snappy means quick and lively — or irritable in reply. The word has a bright side and a sharp one.
The bright sense describes style and pace, so a snappy title, snappy dialogue and a snappy dresser are all compliments. The instruction make it snappy means hurry up. The other sense comes straight from a dog snapping, so somebody being snappy is short-tempered, usually because they are tired or hungry. Which one you get depends entirely on what the word is attached to.
Dutch
snappen
English
snap
English, today
snappy
From snap + -y. snap Borrowed from Dutch snappen. From Dutch snappen (“to bite; seize”) or Low German snappen (“to bite; seize”), ultimately from Proto-West Germanic snappōn, from Proto-Germanic snappōną (“to snap; snatch; chatter”), intensive form of snapāną ”to snap; grab”, whence Old Norse snapa (“to get; scrounge”), from Proto-Indo-European snep-; compare Lithuanian snãpas (“beak, bill”). (One alternative hypothesis links the Germanic words to snu-, an root deriving words meaning “nose”, “snout”, “sniff” etc., but this is phonetically unsound.) In any case influenced by imitative of the sound; note expressions such as snip-snap, containing the formally unrelated snip. Cognate with West Frisian snappe (“to get; catch; snap”), German schnappen (“to grab”), Swedish snappa (“to snatch”).
Etymology from Wiktionary, CC BY-SA