Trom

In Serbian and Croatian we have adjective "trom" which means:

moving with difficulty
sluggish
slow

It is implied that the slowness and sluggishness comes from heaviness. The word has an opposite meaning to words meaning springiness, lightness of movement, lightness.

In Makedonian we have word "tromav" which means cumbersome, clumsu. 

This word is, as far as i am aware, found only in these South Slavic languages.

And in Celtic languages.

Irish "trom"

of great weight, of high specific gravity, of heavy texture, stodgy, hard to digest, dense, thick, abundant, of great force or intensity, laborious, burdensome, grievous, severe, harsh, tyrannous
unsparing, sultry, oppressive, weighty, profound, important (heavy), dull, tedious, laboured, drowsy, deep, slumberous, oppressed, sad

Old Irish "trom"

heavy (weight), heavy, severe, grievous, difficult, sad, sorrowful, great, vast, powerful, mighty

Scottish Gaelic "trom"

heavy, hard, difficult, weighty, serious, depressed, melancholy, addicted (heavily into something), pregnant (with child)

Manx "trome"

heavy, substantial, dense, difficult, emphatic, intense, pregnant

Welsh "trwm"

literally and figuratively heavy

Apparently all these Celtic words come from Proto Celtic root "trummos" meaning heavy. 

Where does the Serbian word comes from then? 

I personally believe that it is a borrowing from Celtic languages. But how come we find this word only in Celtic languages and in Serbian (Croatian)? When was this word borrowed into Serbian (Croatian) and where?