Why is "Onus" in the Dative Case?

Question

North & Hillard Ex. 211: a general addresses his soldiers as an approaching enemy is about to encircle them.

The following is to be translated into Latin:

"But since the enemy are already upon us, and we have not collected sufficient provisions, if there should be any here faint-hearted, or any that careth not to fight to the death in his Majesty's cause, let him depart, and not be burdensome to us in the siege."

The Answer Book gives:

"cum vero hostes iam adsint neque nos satis frumenti comparaverimus, si quis timidus adest (aut) si quis non animo paratus est usque ad mortem pro rege nostro pugnare, abeat neve nobis hic obsessis oneri sit."

GRAMMATICAL POINT:

In "...abeat neve nobis hic obseesis oneri sit.", the siege is about to begin therefore a future tense is required. There is a supine, "obsessum", from verb "obsideo" (= to besiege), in the dative plural, "obsessis" = "to the besieged ones". A verb of motion with a supine gives "in order to" creating a future tense.

LITERAL TRANSLATION (from "abeat"):

"...let him depart (abeat) in order that he (hic) may (sit) not (neve) 'be burdensome' to us the-besieged-ones (nobis obsessis).

The English version asked for adjective, "burdensome" (= "onerosus"); (N & H), in the Latin, gave the noun, "onus", in the dative case, "oneri" (= "to/ for the burden").

Why? How does "oneri" fit in?

Answer 1

The oneri is a dative of purpose or dativus finalis. A simple example of such a dative is id mihi usui est, "it is of use to me". There are often two datives: the beneficiary and the beneficial thing itself. The other dative can be seen as a dativus commodi.

As a whole, this is known as the double dative construction. The two datives play different roles.

In the context of the end of that sentence, you have essentially ille nobis oneri est. (I have stripped unnecessary things like conjunctive mood and most words and supplied an explicit subject.) Here he (ille) is (est) burdensome (oneri) to us (nobis).

You could also say ille nobis onerosus est, and that would parallel the English construction much more closely. Two important points: (1) The book only offers a translation to Latin, not the translation. You have choices. (2) Translation is about understanding a story and retelling it in another language. Don't look at it word by word. Good translations work thought by thought instead.