Hint with an alternative definition: it is the threshold of a doorway (first definition) — like a laid «go-rope» or a porch in front of the door of barrack or any house (you have to cross it if you want to enter) / grand rapids, rifts of a river (second definition) / the end-point of movement or process (third definition) — point + rock (no way to go after it) 

Hint with composition: port + rock + bar (shape)

Hint with translation + consonant words (by sounds or letters): it is like a “prologue” to the house

Russian pro-tip: у двери на полу рог лежит поперёк

Bonus hint: порог (г — от ног) — порох (х — от полыхает, бах, ох!) — порок (к — от рока, корысти, конца) = чтоб порок не переступил порог используй порох!

Mental visualization: П (door sill) + ОР … ОГ (sills and rugs)