Monday hammer
n.— «Professor Sargant Florence still seems surprised that absenteeism should be at its highest on Mondays.…From its relatively greater incidence among unskilled men, he asks whether it is due to boredom, and does not discuss the possiblity that it may be due to sheer physical incapacity (on Clydeside a hammer with a very thick head is known as a “Monday hammer”).» —“Review: Economics and Sociology of Industry. A Realistic Analysis of Development.” by Colin Clark Economic Journal (U.K.) Mar., 1965. (source: Double-Tongued Dictionary)