A correspondent writes to ask about the premodifying use of couple without a following of. She has found a usage comment in her Canadian Oxford Dictionary which says that this usage 'is highly informal and should be avoided in writing', whereas her Merriam-Webster states that the usage 'has been called non-standard, but it is not'. "Why has the usage developed?', she asks.
Couple of goes back to the 14th century, whereas the of-less construction seems to be relatively recent. The OED first recorded usage is 1925, and tags the expression as 'US colloquial': 'a couple months in Italy', 'a couple hundred'. The American origin is enough to explain British caution, and thus the difference in attitude of the two dictionaries.
But where did the usage come from? I opt for a phonological explanation. The reduction of of resulted in coupla, usually written that way (as also cuppa tea and suchlike) . That has a first recorded usage of 1908. It would have been a short step to elide the vowel completely. Reinforcement may then have come from the later usage couple more, where of is disallowed, as in: 'Wait a couple more minutes' (cf also 'Wait a couple minutes more'). That began to appear in the 1930s.
The Merriam-Webster comment suggests that the usage has become increasingly accepted in the US, but not everyone agrees. It's made very little headway in the UK, so far.