Monday hammer n. jocularly, a (heavy) sledge or hammer (said to be appropriate for use on a Monday). (source: Double-Tongued Dictionary)
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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.