slug
Massexact by definition
The slug is the mass that one pound-force accelerates at one foot per second squared, which makes it exactly 14.5939029372064 kilograms, or about 32.174 pounds. It exists so that F = ma can be written in customary units with no conversion constant in it at all.
Watch out: The slug is a mass, the pound-force is a force, and the pound is a mass. Air density in US aerodynamics is 0.002377 slug/ft³ at sea level, the same air as 0.0765 lb/ft³; the two numbers differ by the factor 32.174 and swapping them silently is a classic wind-tunnel error. The competing convention keeps pounds for mass and carries g_c = 32.174 lb·ft/(lbf·s²) through every equation instead. Pick one and stay in it.
| 1 slug | 14.593903 kg |
Proposed around 1900 by Arthur Mason Worthington and adopted slowly, mostly by aerodynamicists. The name is a joke about something heavy and sluggish, and it stuck because the alternative names on offer were worse.