pentathlon

Borrowed from Ancient Greek πένταθλον (péntathlon), a fusion of πέντε (pénte, “five”) and ἆθλον (âthlon, “contest”). Its structure is transparent: five contests, bundled into one word.

The facts here don't trace a shift in meaning — the Greek term already named a five-event contest, and English simply borrowed it whole.

The second half, ἆθλον (âthlon, “contest”), is also the root behind English 'athlete' and 'athletic', words for those who compete.