motashakker-am looks like the active participle of the Arabic form V verb (تَفَعَّلَ) with the Persian 1st person singular of the verb "to be": "I am thankful"; tashakkor is the masdar (verbal noun) of the same verb form. Form V of a verb is usually reflexive and intransitive, so while form I of the verb shakara means "to thank", form V tashakkara means "to be thankful".Do they come from Arabic?
First point: Some if not most nouns are with -t, as in muHabbat/maHabbat but there are instances of the noun remaining with -ah ending, e.g iraadat/iraadahI thought that the تاء مربوطة of a feminine nisba used as an abstract noun becomes -t in Udru, e.g. علمیت، انسانیت etc. So shouldn't we expect شکریت if it means "thankfulness"?
Slightly off-topic: in Hindi this abstract noun suffix is pronounced and spelled with a single y, इंसानियत, and I've heard this in Urdu as well (the spelling is of course ambiguous without diacritics). Is this the standard pronunciation in Urdu, or do high registers double the y, -iyyat?
Great explanations and suggestions with which I can wholeheartedly agree. There is a phrase آدابِ شکریّہ aadaab-e-shukriyyah which I believe gave origin to the stand-alone word shukriyyah.[...] Back to shukriyah, it looks a lot like a feminine nisba shukriyyah of the form I masdar شُكْرٌ . Maybe this was used in accordance with the rule that non-human plurals get feminine singular adjectives? I.e. shukriyyah is short for "[words, tokens, expressions etc.] of thanks". I believe izafe constructions with a plural first member and singular feminine adj. are quite common in Persian: e.g. قرون ماضیه , گناهان کبیرہ. Maybe shukriyah was originally part of an Indo-Persian phrase of this type? [...]šukran is the accusative of šukr “thanks”; mutašakkir is a participle “thanking”; šukriyya is the feminine singular of the adjective šukrī “thankful”, but it can be used also as an abstract noun “thankfulness”. All these words are Arabic, but only the first is used to mean “thank you” in Arabic.