"And chamushim [armed/equipped]" - the word chamushim means nothing other than armed, as it is said (Joshua 1:14), "and you shall cross over armed." This teaches that they were distinguished by five kinds of weapons [reading chamushim as related to chamesh, five]. Another interpretation: chamushim means nothing other than girded for action, as it is said, "And the children of Gad and the children of Reuben crossed over armed, about forty thousand equipped for the army" (Joshua 4:12-13).
What It Means That Israel Went Up Chamushim From Egypt
Yalkut Shimoni on Torah 227:6
וַחֲמֻשִׁים, אֵין חֲמֻשִׁים אֶלָּא מְזוּיָּינִים, שֶׁנֶּאֱמַר (יהושע א, יד) "וְאַתֶּם תַּעַבְרוּ חֲמֻשִׁים" מְלַמֵּד שֶׁהָיוּ מְצוּיָּינִין בַּחֲמִשָּׁה מִינֵי זַיִן, דָּבָר אַחֵר, אֵין חֲמֻשִׁים אֶלָּא מְזוֹרָזִים, שֶׁנֶּאֱמַר "וַיַּעַבְרוּ בְּנֵי גָּד וּבְנֵי רְאוּבֵן חֲמֻשִׁים כְּאַרְבָּעִים אֶלֶף חֲלוּצֵי [הַ]צָּבָא".