Now, all these years later, I started wondering - does shukran have a Hebrew cognate? Indeed it does. Stahl writes that shukran is related to the Hebrew word שכר sachar, meaning wages or reward. The verb שכר means "to hire, rent", and Klein also shows the connection to Arabic:
Phoenician שכר (=to hire), Ugaritic shkr (= to let out on hire, let), Arabic shakara (he rewarded, thanked), Ethiopian shekar ( = hired)The name of the singer Shakira is also related, as it comes from the Arabic meaning grateful or thankful.
Continuing with the etymology of שכר, Ben Yehuda suggests that perhaps it was originally the shafel form of the Aramaic אגר or the Hebrew כרה. The root אגר means "to hire", and is familiar to us from the Mishnaic phrase לפום צערא אגרא - "according to the suffering is the reward." According to Klein, this is the root of the word אגרה agra - the fee for government services in Israel, and may also be the source of the word אגורה agora - a coin, now meaning one hundredth of a shekel.
The root כרה means "to buy, trade", and is related to the Arabic kara - "he let for hire".
One more root that may be connected is חכר, meaning "to lease". Klein writes that it is probably related to שכר.
Knowing that shukran is related to שכר may help me (and others) with a common mixup that English speakers have when speaking Hebrew. In English, "rent" is one of those rare verbs that can be used in more than one direction - he rented the apartment to a tenant, he rented the car from the company. In Hebrew however, the verbs are clearly different לשכור is to "rent from", whereas להשכיר is to "rent to". Knowing that shukran meant to give thanks - i.e. to give reward - makes it easier for me to remember that the verb לשכור means to give reward, to give payment.
I can think of one other English verb that has this same phenomenon - "to nurse". A mother nurses a child, a child nurses from a mother. And again in Hebrew, there are two different verbs - ינק and הניקה. And here too, I've heard English speakers get them mixed up when speaking Hebrew. Can anyone think of other examples? Shukran in advance...