Reiectis = re + iacio = throw back or away. The context will tell you which is the better meaning for re-. Notice the force of all prefixes in composition, whether separate or inseparable as here. For re-, see [pp. 280], [281].
pilis = the pīlum, the distinctively Roman missile weapon.
cominus = comminus: i.e. con (= cum) + manus = hand to hand. N.B.—In composition a often becomes i, cf. iacio, re-icio; and cf. e-minus = at a distance.
(ii.) Translation.—
PUGNATUM EST. The only finite verb in the sentence, and the principal one. The form shows you it is a so-called impersonal verb, and therefore the subject must be sought from the verb itself in connection with the context. Here, clearly, you must translate the battle was fought.
cominus tells us how, i.e. hand to hand.
reiectis pilis. You will recognise this as an ablative absolute phrase. But do not translate this literally their javelins having been thrown away, for this is not English. Let the principal verb and the sense generally guide you to the force of the phrase. Thus you can see here that the Roman soldiers had no use for their javelins, and so threw them away as a useless encumbrance. (The context tells us that the Roman soldiers had no time to hurl their javelins against the foe.) You can now translate the whole sentence—(and so) the Romans threw away their javelins and fought hand to hand with swords.
[II.] At Germani celeriter, ex consuetudine sua, phalange facta, impetus gladiorum exceperunt.
(i.) Vocabulary.—
ex consuetudine sua = according to their custom. You will probably have met with consuetudo, or consuesco, or suesco. Our own word custom comes from it through the French coutume. For this use of ex cf. ex sententia, ex voluntate.