The word G. amosgarn “robin” appeared in Gnomish Lexicon Slips modifying the Gnomish Lexicon of the 1910s (PE13/110), a combination of G. ammos “breast(plate)” and G. carn(in) “scarlet” and equivalent to ᴱQ. karneambar “robin, (lit.) red-breast” (QL/48).
Conceptual Development: Several similar forms appeared in the Gnomish Lexicon proper: G. {ambogrintha, amrintha >>} ammogrint “red breast” (GL/19) and G. {crinthambos >>} crinthammos “red breast, robin” (GL/27), but these words used G. crintha “rosy, pink” rather than G. carn(in) “scarlet”.
Neo-Sindarin: For purposes of Neo-Sindarin, I would adapt this word as ᴺS. amosgarn “robin, (lit.) red-breast” as a combination of ᴺS. ammos “breast” and a modified form of S. caran “red”.
A word for “robin” in the Gnomish Lexicon of the 1910s with a variant cílobi, related to the verb G. ciloba- “chirp” (GL/26) and ᴱQ. kilapi “robin” from the contemporaneous Qenya Lexicon (QL/46).