Why is M used as an abbreviation for thousands?

Answer 1

The letters which represent numbers in the Roman system are in many cases the first letter of the Latin word for the number. eg C = 100 = centum M = 1000 = mille

Answer 2

M is not an abbreviation for 'thousand' - but the Roman symbol for it.

The Romans and Greeks didn't have modern numbers, so they used letters of the Alphabet for writing numbers down (and for reckoning with).

These days a modernised system of Roman numerals is used for writing down the year of construction for a building (and sometimes in copyright notices). You also see them on cabinet clocks.

On a cabinet clock the numbers go: I II III IV V VI VII VII IX X XI XII.

On a building you might see: MDCCCLVII

M: 1000

D: 500

C: 100 (x3)

L: 50

V: 5

I: 1 (x2)

::

1000 +500 + 100 (x3) + 50 + 5 + 1 (x2) = 1857