From Longman Dictionary of Contemporary Englishthanks a lotthanks a lot informal (also thanks a bunch American English spoken) used when really you are annoyed about something and you do not mean ‘thank you’ at all → thanks
Examples from the Corpus
thanks a lot• "I forgot to bring your money." "Well, thanks a lot!"• Gee, thanks a lot for saving me from Madonna in the morning.• Hey, thanks a lot, Wyatt. 