thrifty, n. — Green’s Dictionary of Slang

thrifty n.

[the twelve-sided coin, minted only 1937–52, carried a picture of the plant thrift on its reverse + play on SE]

a threepenny bit.

1937		Nott. Eve. Post 30 Apr. 6/3: The introduction of the new twelve-sided threepenny-bit started a new popular game — that of finding a nickname for the coin. ‘Thrifty’ was a frequent suggestion.
1984		Partridge DSUE (8th edn) 1225: thrifty. A threepenny piece, esp. the 12-sided one minted 1937–52, which bore a representation of the plant thrift on the reverse.