Lincoln was an attorney talking to a group of law-makers. Everybody knew what the word meant.
DEPORTATION, civil law. Among the Romans a perpetual banishment, depriving the banished of his rights as a citizen; it differed from relegation (q. v.) and exile. (q. v.). 1 Bro. Civ. Law, 125 note; Inst. 1, 12, 1 and 2; -Dig. 48, 22, 14, 1.
Bouvier's Law Dictionary, 1856 edition
From a more modern law dictionary:
Deportation
Banishment to a foreign country, attended with confiscation of property and deprivation of civil rights. A punishment derived from the deportatio (q.v.) of the roman Law.
The transfer of an alien, excluded or expelled, from the United States to a foreign country. Petition for Milanovic, D.C.N.Y., 162 F.Supp. 890, 892. The removal or sending back of an alien to the country from which he came because his presence is deemed inconsistent with the public welfare, and without any punishment being imposed or contemplated. The list of grounds for deporation are set forth at 8 U.S.C.A. § 1252-1254. See also Banishment. Compare Extradition.
Black's Law Dictionary, 6 Ed.
Did they? How about a contemporary law dictionary, or dictionary of any kind? Every time Lincoln uses the word 'deport' it seems to be included with voluntary colonization plans. In the immortal words of Inigo Montoya, "You keep using that word. I don' think it means what you think it means."