The Spanish word 'ejército' (meaning 'army') comes from the Latin word 'exercitus', which meant 'trained group' or 'army'. 'Exercitus' developed from the Latin verb 'exercere' meaning 'to exercise' or 'to train', which itself was formed from combining two Latin elements: the prefix 'ex-' meaning 'out' and 'arcere' meaning 'to enclose or contain'. The combination of these elements evolved to suggest the idea of training or exercising, likely from the concept of 'working out' or 'bringing forth' one's abilities, which then led to describing a trained group of soldiers.

Several common Spanish words share this Latin root, including 'ejercicio' (exercise), 'ejercer' (to exercise, to practice), and 'ejercitar' (to exercise, to train). All these words maintain the core concept of activity, practice, or training that was present in the Latin original.

English speakers can easily connect 'ejército' to several familiar English words that come from the same Latin root 'exercere'. The most obvious ones are 'exercise' and 'exert', which maintain the sense of putting forth effort or training. The word 'exercise' is particularly helpful to remember since it shares both the etymology and a similar sound pattern with 'ejército', though they've evolved to have different specific meanings.