The Hebrew word for the number five is חמש - chamesh. Another set of words that would seem to have the same root are chamush חמוש - armed and tachmoshet תחמושת - ammunition. Is there a connection between them?
The earliest source that might provide an answer is Shmot 13:18 - וַחֲמֻשִׁים עָלוּ בְנֵי-יִשְׂרָאֵל, מֵאֶרֶץ מִצְרָיִם. "Bnei Yisrael went up, chamushim, from the land of Egypt". Most translators and commentaries explain chamushim here as meaning "armed". (There are some exceptions. The Septuagint translates chamushim as "the fifth generation", despite translating chamushim in Yehoshua 1:14 as "armed". Apparently the unusual translation is due to the difficulty explaining the source of the armaments. And as far as the midrash that Rashi quotes, that only one fifth of Bnei Yisrael left Egypt, it is appropriate to quote Ibn Ezra's response: "We have enough trouble explaining to the Arab scholars how 55 males could have 600,000 males over the age of 20 within 210 years - and there's even more when you include women and children!")
So is there a connection between the two meanings of חמש? Klein believes that there may be. In his entry for חמוש, he writes:
Of uncertain origin. It is possibly related to חמשה (= five), and refers to the division of the army into five parts: van, body, rear and two wings. Hence related to Arab. hamis (= army; properly 'army divided into five parts') from hams (= five.)
Steinberg, in his Milon HaTanach, says that חמש in Arabic means "collection and connection", and this is the source of the number chamesh, which means "clenching of the five fingers". He gives examples from other languages where the same word means hand and five, for example piast' and piat' in Russian. He therefore explains the meaning of chamush as armed, because the soldiers are gathered and collected together. (We saw something similar in our explanation of the connection between lechem and milchama.)
I thought perhaps there was a similar development in English, between arm (limb) and arm (weapon), but although they come from the same Indo-European root, there doesn't seem to be the same type of sense development as in Hebrew.