Strengthful is from strength + -ful. Strength is from Old English strengþu, from strang ('strong'), from Proto-Germanic *strangaz ('tight, strong'). Important recorded forms include strengþu (Old English) and *strangaz (Proto-Germanic). Morphologically, -ful marks possession, abundance, or tendency, so the word names something characterized by the base idea. The modern sense continues the older lexical line without requiring a separate origin. In current use it covers full of strength; powerful (archaic). The wider family includes derived forms such as strengthfully and related words such as strength, strengthen, and strong.