The Thracian word ostas
Word ostas
Meaning river mouth

Other words that might be of interest
achel- ‘water’, ala ‘current, stream’, alta ‘current, stream’, arma ‘swamp’, ars ‘to flow’, arta(s) ‘river’, e(i)b ‘to flow’, iuras ‘water, river’, kaba(s) ‘bog, swamp’, kalsas ‘dry, dried up’, mar ‘water, river, bog’, pala ‘swamp’, sara ‘river’, sind(u)- ‘river’, strumon ‘river’, opa ‘water’, krisos ‘river name’