Colloquialism for the selector lever on a select-fire firearm or safety on an automatic weapon, which engages the mechanical safety, semi-automatic fire, or automatic fire/3 round burst. Switching to automatic fire and putting streams of lead downrange is well known to elicit giggles of joy from anyone who grew up playing video games or watching GI Joe/The A-Team.
1) Private Joe Snuffy flipped the giggle switch on his M-249 for the first time, and laughed maniacally as he engaged the pop-up targets on his qualification range.

2) Rangemaster: "Flip that giggle switch and give 'er a whirl

Trainee: "heh heh heh . . ." *BRRRRRRRRAP!*
by DoomsdayJesus October 3, 2013
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