This is a misspelling; the correct spelling is 'careful,' meaning paying close attention and avoiding mistakes or dangers.
Formed from 'care' (Old English 'caru,' meaning worry or concern) plus the suffix '-ful' (meaning full of). The double-l spelling was common in Middle English but is now considered non-standard.
You've found a spelling variation! In historical documents, you'll see 'carefull' with two L's because spelling rules weren't standardized until printing became common and dictionaries were created.
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