As part of his online Grammar of the Polish Language, Grzegorz Jagodziński has a list of Polish etymologies, a table of numerals in some of the main IE languages, and a detailed discussion of the etymology of the Polish (and other IE) numerals, the last-named perhaps the most interesting; here’s one of the shorter sections:
PS †pętь;, originally a numeral substantive *penkʷtis (Skr. paŋktiṣ; ‘the number five’) from the proper numeral *penkʷe (Slavic languages have preserved only the numerals 1-4, cf. Gr. pénte, dial. pémpe, Lat. quīnque <*kʷenkʷe with assimilation; the contrary assimilation can be observed in Goth. fimf < *pempe, and surely in Gr. pémptos < *penkʷtos, because *kʷ before a consonant developed into k in this language under normal conditions). In the collective form pięcioro the formant –er– is present. If it is transferred from czworo, it must have happened as early as in Balto-Slavic, cf. Lith. penkerì.
The numeral pięć is connected to the substantive pięść < †pęstь;, cf. Germ. Faust, Engl. fist < †funxsti– < *pn̥kʷ-sti- (originally ‘hand’; the Slavic form can, even if need not, come from the root with full vocalism), cf. also Engl. finger < *pn̥kʷ-r-. From the same stem, piądź, piędź < *penkʷ-dhi- ‘span, inch of ground’ seems to originate, or we can have the related stem *pendh- here. Connections with Gr. pygmḗ; and Lat. pugnus ‘fist’ (<*pug– < ? *pogʷ-) would also be possible, at least in the distant past.
An interesting problem is caused by Lith. kùmštis, Prus. kuntis ‘fist’ < *kumpstis < *punkstis (metathesis) < *pn̥kʷ-sti-. however we can see further connection also to Ltv. kàmpt ‘grab, catch’, and yet further to Lat. capere ‘catch’ (probably from there Engl. keep) and PG †xabē– (cf. Engl. have). Perhaps the same stem, but with irregular phonetic changes, is present in Lat. habēre ‘have’ < *ghəbh– ~ *kəp-, cf. also modern Pol. nagabywać ‘to ply, to molest, to importune’ and OPol. gabać ‘to attack’, Lith. góbti ‘to take possession of sth.’ < *ghōbh-. An obstacle for a reconstruction of Proto-IE stems of different words meaning ‘5’, ‘hand’, ‘catch’, ‘take’ and ‘have’ is the difference of the velar kʷ ~ k (gh). We must not forget, however, that we may talk about a very distant relationship only, and during thousands of years many irregular changes might have occurred.
Lots of fun for anyone interested in Slavic and Indo-European. (The numeral etymology page via aldiboronti at Wordorigins.)