This word has an uncertain classification
The origin of this word is unknown
‘to, by, at’
Attic Greek: θηλή ‘teat, nipple’
The gloss is very doubtful regarding its meaning (does it refer to a womans sinus?) and origin (no commentary on its origin). Hoffmann (1906) suggests it is a Thraco-Phrygian loan consisting of ΑΔ- + θήλα "breast". It could be simply Macedonian δ ~ θ. Based on the information available, this word should remain unclassified.
See Hoffmann 1906, Filos 2015