Sentinel=see at entry.Guard or Sentry.
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senti"nel" (SA bowler) gets senti when he bowls a beamer and watches over the batsman so as to prevent a boundry on the next ball in case the batsman gets aggressive.. so he is a sentinel !!
A SENTINEL is like a CONSTABLE - a soldier posted to keep watch and guard something.
sena at tunnel
sentinel: sent+in+el(elowd/allowed): we employed a watch man/gaurd at my wedding so that only people who has applied SENT will be ALLOWED (EL) IN inside.
sentinal prime : from transformers was keeping guard on the energon