ETYMOLOGY
from slim (adj.) + slack (adj.)
EXAMPLE
“…Thou planter of Amfrisus, inhabiter of Parnassus, louer of Helicone; lord of fount Caballyne, patrone of the Laurell, inuentor of the Harpe, master of Astrologie, and prince of Phisicke, helpe these poore slimslacks, who haue need of internal remedies, for the restitution of their disturbed braine, destitute witte, obfuscate vnderstanding, lost memorie: …”
From: translation of The Hospitall of Incurable Fooles
By Tomaso Garzoni, 1600
Lexicophilia 