By surface analysis, летя́ (letjá, “to fly”) + -еж (-ež).
лете́ж • (letéž) m
- flight
- “летеж”, in Речник на българския език [Dictionary of the Bulgarian Language] (in Bulgarian), Sofia: Bulgarian Academy of Sciences, 2014
- “летеж”, in Речник на българския език [Dictionary of the Bulgarian Language] (in Bulgarian), Chitanka, 2010