Now, before you report me for libeling the “boys in blue,” just trust me for a little while. Imagine that your are an officer with the task of getting cars to slow down in front of, say, a school. What more effective way would their be than to physically force the cars to slow down? Of course, you would lie across the road and endure a few bumps for the sake of the children. Thus, the idea of a lying policeman was born in Russia. After a while, the policemen started complaining that it was hard to write reports while lying across the street and they were getting too smushed to slow the cars down well, so someone had a bright idea to make big bumps in the road out of asphalt. However, the name stuck and now we refer to such an asphalt bump or speed bump as a “lying policeman” (лежачий полицейский).

The first word in this phrase, лежачий, is an adjective that formed from the verb лежать. Thus, it can be used to describe different nouns, and here are a few more examples in some other well-known phrases.

лежачее место – sleeping berth, bunk

лежачий больной – bedridden patient; bed patient (Лежачий пациент требует особого ухода. (A bedridden patient requires particular care.); Лежачий пациент приподнял голову от подушки. (The bedridden patient raised his head off the pillow.)

There’s also an apropos proverb to remember when your brain hurts from studying Russian:

под лежачий камень вода не течёт “nothing seek, nothing find” or “no pains, no gains”

And finally:

лежачего не бьют “don’t hit a man when he’s down”