It has a very clear definition. It's just been obscured by the English translation.If modest can mean whatever we want then it can be anything or nothing; the command is made null.
The word is "kosmios" in Greek (G2887), and appears twice in scripture, translated as "modest" or "of good behaviour" and applied to both men and women:
1Ti 2:9In like manner also, that women adorn themselves in modest apparel, with shamefacedness and sobriety; not with broided hair, or gold, or pearls, or costly array;
1Ti 3:2A bishop then must be blameless, the husband of one wife, vigilant, sober, of good behaviour, given to hospitality, apt to teach;
Definition:
κόσμιος, κόσμον, of three term. in classical Greek, cf. WHs Appendix, p. 157; Winers Grammar, § 11, 1; (Buttmann, 25 (22f)) (κόσμος), well-arranged, seemly, modest: 1 Timothy 2:9 (WH marginal reading κοσμίως); of a man living with decorum, a well-ordered life, 1 Timothy 3:2. (Aristophanes, Xenophon, Plato, Isocrates, Lysias, others) (Cf. Trench, § xcii.)
The meaning is very clear, and the instructions in both 1 Timothy 2:9 and 3:2 are sound and not nullified in any way by this discussion.