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.)