The river name is attested only in Dionysius Periegetes (Dion. Perieg. 575).

The appellations are ancient: the tribe is mentioned only by Herodotus; Apsinthis, according to Strabo, is an old name for the region of Corpilice; and the settlement name Apsinthus, mentioned only by Stephanus of Byzantium, is, according to Hirschfeld, an old name for Enos, which is uncertain; it is more likely that the homonymous settlement was located along the river. The river name is also a hapax and belongs to an ancient onomastic layer: as early as in Herodotus (7, 58) the river is already called Melas. Dionysius Periegetes, who is the only one to call it Ἂψινθος, wrote his poem in the 2^nd century, but drew his geographical knowledge mainly from Eratosthenes and Posidonius. Although tracing the name in more ancient sources would be highly hypothetical, it is most likely to be found in one of the logographers, Herodotus’ predecessors. 

The name was defined as Thracian by D. Detschew, who denied any connection with the name of the plant ἀψίνθιον ‘wormwood’, comparing it with the Illyrian river name Apsus and even considering it possible that the name of the river in Illyria was of Thracian origin.

In the extensive discussion on the name that developed over the following decades, three groups of opinions emerged:

1) scholars who consider the hydronym and other names with the same root to be Thracian, denying any connection with the pre-Greek appellative ἄψίνθος (D. Dechev, I. Duridanov, P. Dimitrov)

2) those who believe that there is a connection with ἄψίνθος, but deny the Thracian origin of the hydronym, toponym and ethnonym and consider them to be Greek (Venedikov) or Pelasgian (G. Mihaylov)

3) those who accept that the case represents a lexical parallel between Thracian and Pelasgian (L. Gindin, Yu. Otkupshtikov, S. Yanakieva).

Among the etymologies, the most convincing is that of I. Duridanov, according to whom the river name derives from the IE *apsa ‘aspen’. The formation of river and settlement names from plant names is a widespread phenomenon in toponymy, and in this case, the modern name of the river – Kavak-çayı/Kavaksuyu, which can be considered a translation of the Slavic name, which in turn comes from the Thracian one, also supports such interpretation. A connection with the name of the plant ἄψίνθος, ἀψίνθιον ‘wormwood’ is entirely possible, since in Indo-European languages it is common for plant names to be derived from the names of trees (such as willowherb from willow). Etymologically, both the toponym and the appellative are probably derived from the word for ‘aspen’ (‘willow’), in which case ἄψίνθος, ἀψίνθιον ‘wormwood’ would literally mean ‘aspen’ and the river name Ἂψινθος would mean “Aspen river” or ‘Poplar river’. The relationship with the appellative ἀψίνθιον does not contradict the Thracian origin of the river, settlement and tribal name, because the word ἀψίνθιον is most likely of pre-Greek origin, and there is a very close relationship and lexical similarity between the Thracian language and the pre-Greek substratum, so that this word and/or the word for ‘willow’ from the same root did probably also exist in Thracian.