From Middle English waccheful, equivalent to watch +‎ -ful. Compare West Frisian waaks (“watchful”), Dutch waaks, waakzaam (“watchful”), German wachsam (“watchful”), Swedish vaksam (“watchful”).

• (Received Pronunciation) IPA^(key): /ˈwɒt͡ʃ.fəl/
• (General American) IPA^(key): /ˈwɔt͡ʃ.fəl/
• Hyphenation: watch‧ful

• Audio (US):

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watchful (comparative more watchful, superlative most watchful)

1. Fully observant, vigilant, or aware.

   Synonym: jealous

   The teacher kept a watchful eye on her pupils during the school trip.

   • 1954 July 29, J[ohn] R[onald] R[euel] Tolkien, “The Shadow of the Past”, in The Fellowship of the Ring: Being the First Part of The Lord of the Rings, New York, N.Y.: Ballantine Books, published September 1973, →ISBN, page 93:

     But there was so much at stake that I had to take some risk—though even when I was far away there has never been a day when the Shire has not been guarded by watchful eyes.

• ever-watchful
• watchful waiting
• watchfully
• watchfulness