Learn English – Origin of the phrase “Mustard on the beat”

http://www.phrases.org.uk/meanings/cut-the-mustard.html

There has been an association between the heat and piquancy of mustard and the zest and energy of people's behaviour. This dates back to at least 1672, when the term 'as keen as mustard' is first recorded. 'Up to mustard' or just 'mustard' means up to standard in the same way as 'up to snuff'. 'Cutting' has also long been used to mean 'exhibiting', as in the phrase 'cutting a fine figure'. Unless some actual evidence is found for the other proposed explanations, the derivation of 'cutting the mustard' as an alternative way of saying 'exhibiting one's high standards' is by far the most likely.

Whatever the coinage, the phrase itself emerged in the USA towards the end of the 19th century. The earliest example in print that I've found is from The Iowa State Reporter, August 1897, in a piece about the rivalry between two Iowa towns:

Dubuque had the crowds, but Waterloo "Cut the Mustard"

The use of quotation marks and the lack of any explanation of the term in that citation imply that 'cut the mustard' was already known to Iowa readers and earlier printed examples may yet turn up.

As the link to Phrase Finder above explains, this phrase evolved from the phrase filthy lucre from the 1500s. Here's what Etymonline has on lucre:

lucre (n.)

late 14c., from L. lucrum "gain, advantage, profit; wealth, riches," from PIE root **lau*- "gain, profit" (cf. Gk. apo-lanein "to enjoy," Goth. launs, Ger. lohn "wages, reward," and possibly Skt. lotam, lotram "booty"). Filthy lucre (Tit. i:11) is Tyndale's rendering of Gk. aischron kerdos.

As Phrase Finder explains:

Following on the the term "filthy lucre", money became known by the slang term "the filthy", and it isn't a great leap from there to the rich being called the "filthy rich". This was first used as a noun phrase meaning "rich people; who have become so by dishonourable means".

Phrase Finder claims a 1920s origin, but the two words can be found together in print from the early 1800s. This, however, is the earliest example I can find of it being used as a set phrase rather than as a description of rich people who are physically dirty. Here's a clip from A Certain Rich Man by William Allen White, 1909 (date check):

And a final point worth noting from Phrase Finder:

As time went on the negative associations have softened somewhat. It has become to mean "extremely rich" rather than "dishonourably rich", although there may still be a trace of an unfavourable implication associated with it.