An adjective (and adverb?) for “uphill” in The Etymologies of the 1930s, a combination of am “up” and N. penn “declivity, ✱slope” (Ety/PEN). It was contrasted with N. dadben “downhill, inclined, prone” (Ety/AM², PEN).
Conceptual Development: In The Etymologies it first appeared as N. amdenn, a derivative of ᴹ√DEN “hillside, slope”, but the meaning of this root was change to ᴹ√DEN “hole; gap, passage” (Ety/DEN), after which the form amben < ᴹ√PEN(ED) was introduced (see above). The earliest appearance of this word was in Early Noldorin Word-lists of the 1920s where it was ᴱN. amvenn “uphill; arduous, difficult, tiresome”, marked as both an adjective and adverb, along with a noun variant ᴱN. amvinn “slope, incline, hillside” (PE13/139, 159-160). This early Noldorin form was a combination of ᴱN. am “up” and ᴱN. benn “sloping”.
Neo-Sindarin: Given its Early Noldorin use for “arduous, difficult, tiresome”, amben might be used colloquially in Neo-Sindarin with a similar sense for a thing that is difficult, analogous to English usages like an “uphill battle”: dagor amben.
A prefix for “snake” in The Etymologies of the 1930s derived from the root ᴹ√ANGWA of the same meaning, most notably an element in N. amlug “dragon” (Ety/ANGWA).