uss-

• oss- (GOI)
• od- (before a stop or m)
• ó-, úa- (before a liquid or n)

From a merger of two prefixes.

• One is from Proto-Celtic *uts-, from Proto-Indo-European *úd-s- (“out”).
• Another source is from Proto-Celtic *(o)uxs-, from Proto-Indo-European *ups, a sigmatic variant of *upó (“up”).

uss- (pretonic as-)

1. up, off
2. augment used instead of ro- in a few cases^[1]

   com- + uss- + -techtatar → con·oitechtar

   com- + uss- + -malt → con·omalt

   to- + uss- + -baig → ·topaig^[2]

   to- + uss- + -bacht → ·tópacht

• Thurneysen, Rudolf (1940) [1909] D. A. Binchy and Osborn Bergin, transl., A Grammar of Old Irish, Dublin Institute for Advanced Studies, translation of Handbuch des Alt-Irischen (in German), →ISBN, § 349, pages 525–26; reprinted 2017