The Spanish word 'ejercicio' (meaning 'exercise') comes from the Latin word 'exercitium', which also meant 'exercise' or 'practice'. This Latin word developed from 'exercitare', meaning 'to exercise frequently', which in turn came from 'exercere', meaning 'to drive out, keep busy, or exercise'.

The Latin 'exercere' is particularly interesting as it's a combination of two parts: the prefix 'ex-' meaning 'out of, from' and 'arcere' meaning 'to enclose or contain'. So literally, the original concept was about 'driving out from an enclosed space' - perhaps similar to driving cattle out to exercise them - which eventually developed into our modern notion of exercise and practice.

Some related Spanish words include 'ejercer' (to exercise, as in exercising rights or a profession), 'ejercitar' (to exercise or practice), and 'ejército' (army) - all sharing the same Latin root. The connection to 'ejército' (army) comes from the fact that military troops would perform physical exercises and drills.

English speakers can easily remember 'ejercicio' by connecting it to the English word 'exercise', which comes from the same Latin root 'exercitium'. Other related English words include 'exert' (from Latin 'exercere') and 'exercise'. The similarity between 'ejercicio' and 'exercise' makes it relatively easy for English speakers to remember this Spanish word.