Snell has two unrelated meanings.
A snell is a short length of fishing line that attaches a hook to the main line; to snell a hook is to tie it this way.
Separately, snell is a Scottish and Northern English dialect word meaning bitterly cold or sharp: a snell wind off the moor.
1. fishing — leader, One who guides others; hook (Curved catching device); knot (Tied rope fastening); tippet (Narrow clerical scarf); monofilament · fluorocarbon · terminal · tackle · rigging · line · angler · bait · lure · fly-fishing
2. bitter — biting, Harshly critical or cold; piercing (Penetrating or very sharp); keen (Sharp or enthusiastic); cutting (Dividing via sharp edge); sharp · raw · harsh · brisk · nippy · frigid · stinging · penetrating · icy · chill