Why is the word "pants" plural?

Solution 1:

A quick search led me to the excellent site World Wide Words run by Michael Quinion

The site has an entire page on this issue. Here's a brief snippet.

Before the days of modern tailoring, such garments, whether underwear or outerwear, were indeed made in two parts, one for each leg. The pieces were put on each leg separately and then wrapped and tied or belted at the waist (just like cowboys’ chaps). The plural usage persisted out of habit even after the garments had become physically one piece. However, a shirt was a single piece of cloth, so it was always singular.

Solution 2:

Apparently in the past they were two tube weakly linked, think to current tights. Hence the plural form.