Monday hammer n. jocularly, a (heavy) sledge or hammer (said to be appropriate for use on a Monday). (source: Double-Tongued Dictionary)
Further reading
Ron in Gloverville, South Carolina, wonders about the phrase since hatchet was hammer, which some use to mean “for a long period of time,” as in My family has lived here since hatchet was hammer. Another phrase he’s heard indicating the same thing...
Old. Elderly. Senior. Why are we so uncomfortable when we talk about reaching a certain point in life? An 82-year-old seeks a more positive term to describe how she feels about her age. And: a linguist helps solve a famous kidnapping case, using the...
A ‘monday hammer’ was a three handled hammer that was used in the chain and anchor trade in the Black Country. Its head weighed one cwt and 5 men were required to handle it. The Vicar of Dudley came to Noah Hingley’s of Netherton and was pleased when the leader of the gang told him that “me and me mates lift it up but the Lord sends it down”.
Its was called a Monday hammer because the men told him that “nobody likes mondays either”
“Strike while the iron’s hot” use as heavy blow as possible, and this was possible with the Monday hammer.