From Wikipedia
The slug is an English unit of mass. It is a mass that accelerates by 1 ft/s² when a force of one pound-force (lbf) is exerted on it. Therefore a slug has a mass of about 32.17405 pound-mass or 14.5939 kg.
1 slug = 1 lbf·s²/ft
The slug is part of a subset of coherent units known as the gravitational foot-pound-second system (FPS), one of several such specialized systems of mechanical units developed in the late 19th and the 20th century. The slug was first used in 1902 by Arthur Mason Worthington (1852–1916) in Dynamics of Rotation (OED).
Another name for this unit in early literature is the geepound.
The term metric slug appears as a footnote in the 1967 seventh edition of Marks Standard Handbook for Mechanical Engineers. The term is sometimes collapsed to mug and it is also called the TME (German: technische Masseneinheit, technical mass unit). It is the mass that accelerates at 1 m/s² under a force of 1 kgf. Because 1 kgf = 9.806 65 N, the metric slug is 9.806 65 kilograms. This has also been called the hyl, but there is an alternate definition of the hyl equalling 9.806 65 grams which would make the kilohyl equal to the TME.