deca- (da-)a metric prefix meaning 10, derived from the Latin word for ten, decem. The Greek form deka- is also used, and it has the advantage of avoiding confusion with deci-, one tenth. The...
dam for dekameter, da for deka-
The abbreviation for decameter is "dam", since the abbreviation of deka is da and the abbreviation of meter is m.
abrv.
It's just that it is like a symbol for and.