The Spanish word 'guachimán' is a loanword that comes directly from the English word 'watchman'. This is an interesting example of how English words can be adapted into Spanish by following Spanish pronunciation patterns. The English 'w' sound was replaced with the Spanish 'gu' (which makes a similar sound), and the spelling was modified to match Spanish phonetic rules, with 'tch' becoming 'ch' and the addition of an accent mark on the final 'á' to maintain the original stress pattern.

Since 'guachimán' is a direct borrowing from English, there aren't any simpler etymologically related Spanish words. However, Spanish speakers might be more familiar with other terms for security guard like 'vigilante' or 'guardia'.

The English word 'watchman' is directly related, being the source word. It's a compound of 'watch' (meaning to guard or observe) and 'man'. Other related English words include 'watch', 'watcher', and 'watching'. The connection is very straightforward since 'guachimán' means exactly the same thing as 'watchman' - someone who guards or keeps watch over a place or property.