I always thought that the words amorous, ardent and arduous were linked in meaing, and that the meaning was primarily the first - amor - to love, with an emphasis on passion.
Arduous, is used very often to mean difficult, strenuous, because that is what it means. Yet, I never wanted to make the mistake of characterizing a professional task - say, writing a transportation plan, as a romantic endeavor. A quick look in the dictionary would have saved me much emabarassment for my adruous using collegues. Nevertheless it still provides a nano second of blush, much like another word I cannot use - titilate, however innocent it is.

Amorous
Pronunciation: \a-ma-ras, am-ras\
Function: adjective
Etymology: Middle English, from Anglo-French, from Medieval Latin amorosus, from Latin amor love, from amare to love
Date: 14th century
1: strongly moved by love and especially sexual love 2: being in love : enamored —usually used with of3 a: indicative of love b: of or relating to love
— am·o·rous·ly adverb
— am·o·rous·ness noun

Ardent
Pronunciation: \är-dent\
Function: adjective
Etymology:
Middle English, from Middle French, from Latin ardent-, ardens, present participle of ardēre to burn, from ardor
Date: 14th century
1 : characterized by warmth of feeling typically expressed in eager zealous support or activity 2 : fiery , hot 3 : shining , glowing
synonyms see impassioned
— ar·dent·ly adverb

Arduous
Pronunciation: \är-j-wes, -dyu-, -jü-s\
Function: adjective
Etymology: Latin arduus high, steep, difficult; akin to Old Irish ard high
Date: 1538
1 a: hard to accomplish or achieve : difficult b: marked by great labor or effort : strenuous 2: hard to climb : steep
synonyms see hard
— ar·du·ous·ly adverb
— ar·du·ous·ness noun