The OED lists "habeas" as "short for habeas corpus" and gives a single example from 1879. (1879 SALA in Daily Tel. 26 June, The unterrified man moved himself by habeas to the Fleet.) It doesn't give a part of speech, but in that example it was a noun. Of course, lawyer types use "habeas" in this meaning, both as a noun and as an adjective, all the time. Here are some nouns:

1935 South Western Reporter (2d series) LXXIX. 866 That after certain cases, including the one against relator, were set for trial on September 25, 1928, and after the cases were called on said date for final action, it was made known to the court that the indictments in said cases were returned some years before; that habeases were duly issued but after diligent search defendants could not be apprehended, etc. 1945 U.S. Reports CCCXXV. 839 The motion for leave to file petition for writ of habeas is denied. 1980 U.S. Reports CDXLIX. 140 On federal habeas, this Court held that the Maryland system did not violate the Clause. 1986 U.S. Reports CDLXXVI. 1177 This case, then, falls squarely within the "ends of justice" exception to the general rule forbidding successive assertions of the same claim on habeas. 1989 U.S. Reports CDXCIII. 909 On state habeas, the Circuit Court held, tersely, that "the jury was properly instructed as to all matters and findings that they were required to make, including but not limited to evidence in mitigation of punishment." 1994 U.S. Reports DX. 1146 n.2 Even if Callins had a legitimate claim of constitutional error, this Court would be deaf to it on federal habeas unless "the state court's rejection of the constitutional challenge was so clearly invalid under then-prevailing legal standards that the decision could not be defended by any reasonable jurist." 1996 U.S. Reports DXIX. 146 Witt was a case arising on federal habeas, where deference to state-court findings is mandated by 28 U.S.C. § 2254(d).

And here are some uses as an adjective:

1894 Southern Reporter XIV. 535 Cy was examined before the coroner's jury, on a habeas trial, before the grand jury, and in the circuit court, where he made statements substantially as above set out. 1914 North Western Reporter CXLIV. 894 That the court permitted the witness, Samuel B. Dennison, to read in evidence the stenographic notes of the testimony of one Leonard Busby, as well as of testimony of defendant, given in a habeas proceeding before Judge Landis in Chicago, Ill. 1963 U.S. Reports CCCLXXII. 397 The first was that the denial of state post-conviction coram nobis relief on the ground of Noia's failure to appeal barred habeas relief because such failure constituted an adequate and independent state ground of decision, such that this Court on direct review of the state coram nobis proceedings would have declined to adjudicate the federal questions presented. 1976 U.S. Reports CDXXVII. 118 n.4 In Stout v. Cupp, a habeas proceeding, the court simply quoted the District Court's finding that if the suppressed evidence had been introduced, "the jury would not have reached a different result." 1987 U.S. Reports CDLXXXI. 556 The procedures followed by respondent's habeas counsel fully comported with fundamental fairness. 1997 U.S. Reports DXX. 466 Others came here by way of federal habeas challenges to state convictions. 2003 U.S. Reports DXXXVII. 507 The habeas court also must engage in a painstaking review of the trial record solely to determine if it was sufficient to support the ineffectiveness claim and thus whether it should have been brought on direct appeal. 2003 U.S. Reports DXL. 18 We may not grant respondent's habeas petition, however, if the state court simply erred in concluding that the State's errors were harmless; rather, habeas relief is appropriate only if the Ohio Court of Appeals applied harmless-error review in an "objectively unreasonable" manner.

The OED also lists occurrences of "habeas corpus," in its full form, but only as far forward as 1827. Obviously, we use it all the time in the law, but listing examples of that wouldn't be interesting here.

Next post: Submitting new words! P.S. The first post in this chain has some updates, partly based on recent comments.