The word upaghna is irregularly formed meaning 'an inclined place for leaning or support'.,
It is formed by adding अप् 3.3.76 to the verb हन् preceded by the upasarga उप, then eliding the penultimate अ of ह; this is the irregularity. Then by sandhi rules हन् is changed into घ्नः । This form would have been given by the affix क also, without any irregularity.
The word आश्रय refers to 'proximity, contiguity'. Thus पर्वतोघ्नः 'the declivity of the mountain'; so also ग्रामोपघ्नः 'bordering the village'.
Why do we say 'when meaning support'? Observe पर्वतोपघातः ।,