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1.
noun -- Submachine gun with integral silencer. Machine Pistole made by Heckler and Koch.
That mofiglio was bustin' off caps on his MP5-SD5 all day.
by Hans September 22, 2003
 
2.
The name for Heckler & Koch's infamous weapon. From the MP5 series.

The MP5SD5, chambered for 9mm, is really an MP5 with a pre-built suppressor of excellent quality. It is used most commonly by counterterrorist forces. Terrorists fear this weapon because of its clandestine capabilities and all-around great performance in the field.
"I want one, but it's automatic. -_-"
-me
by Dave February 03, 2004