Rục is a Vietic language spoken by the Ruc people of Tuyên Hóa district, Quảng Bình province, Vietnam.
Rục literally means 'underground spring', and is a critically endangered language spoken by a small ethnic group that practiced a hunter-gatherer lifestyle until the late 20th century.
In terms of derivational morphology, Ruc retains several forms of affixations that have been lost in other Vietic languages like Vietnamese, but their semantics are largely eroded.
The dative prefix pa- of Ruc has been cited by some linguists as supporting evidence for the Austric languages hypothesis.
For examples, (a) kun4 (‘afraid’) → pakun4 (‘threaten’) kɯcit3 (‘to die’) → kacit3 (‘to kill’) The causative resultative prefix pa- is a homonym: rɨmɛk3 (‘cool’) → parɨmɛk3 (‘to cool (something)’) The normalizing infixes -n- and -r- make a noun from a verb: tʰut (‘to stop up’) → tanut3 (‘stopper’) sɘp3 (‘to cover’) → sanɘp3 (‘a blanket’) The quantifying prefix mu- turns numerals into measuring units.