The last straw (that broke the camel's back)
The final additional small burden that makes the entirety of one's difficulties unbearable.
The full version of this metaphorical phrase is 'the last straw which breaks the camel's back', which has an Old Testament sound about it. Searches there produce many references to straw and camels amongst the smiting and begetting, but no 'last straw'.
The Oxford Dictionary of Quotations lists it as a 'mid 17th century proverb', but offers no supporting evidence for that view. The earliest citation that I can find is from The Edinburgh Advertiser, May 1816:
"MR. BROUGHAM remarked, that if it [a tax on soap] were only 3d. a head, or 4d. and 5d. upon the lower orders, yet straw upon straw was laid till the last straw broke the camel's back."
Some authorities suggest that the phrase is a variant on an olde proverb 'it is the last feather that breaks the horse's back'. That may be so. The earliest I can find for that is after 1816 though and, of course, much later than mid 17th century. That's also from The Edinburgh Advertiser, in November 1829:
"It may be very well for Mr. O'Connell, in his own exultation of heart, to ascribe the success of the Catholic Relief Bill to his 'agitation;' but the fact is, that 'agitation' was only the cause of Emancipation in the same sense in which it is true that the last feather breaks the horse's back."
Last but not least there is, or rather was in 1843, a merger of the two phrases. This appeared in The Southport American in October 1843:
"And finally, the 'feather which breaks the camel's back' having been added to Sir Walter's burthen, he was struck down by paralysis, and after lingering a few months, was gathered to his fathers."