Niyat means intention behind some thing. ksi kaam k peechay chupa irada ya khyal. good intention( achi neyat) bears reward (sawab) but unintentional bad thoughts don,t bear any sort of sin (gunnah), intentional bad niyat is sin.
Intention of a believer is better than its acts.(Hadith)

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When your parents reach old age don,t even say Ufffff in front of them....