The phrase originates in education, specifically the custom of indicating every correct answer in an examination with a mark. Hence, “full marks” would be getting everything right. The phrase appears as early as 1852, according to the OED, with a figurative use in 1889: “I was once fifteen hours on the road… By rail I have done it in an hour and three-quarters. Full marks for steam, it may be thought…”
Though “full marks” shows up in an anonymous 1924 New York Times movie review — “the film version of ‘Monsleur Beaucaire’ is entitled to full marks for the splendor of the settings and the extraordinary beauty of the costimes” — I ascribe that to an Anglophile reviewer. Or, at least, the phrase fell out of favor after that and became unfamiliar here. In the U.S., “marks” is a more general term: one can talk of getting good or not-so-good marks, meaning grades. And I judge the modern American equivalent of “full marks” to be “full credit.”
When I wrote about it in 2019, I categorized “full marks” as “On the Radar.” But it has started to get more popular here. Here’s the Ngram Viewer chart.
It actually showed up a week ago in the Times (as I write), from Bret Stephens in an exchange with fellow columnist Gail Collins: “[President Biden has] done a much better job standing up for Ukraine than I had expected he might, and I’ll give him and his national-security team full marks for that.”
Expect to see it pretty widely, pretty soon.