Latin for 'the Teutonic (or German) custom', and refers to a messy method of preserving a corpse so that it could be partly reburied at a distant location.
The corpse was disembowelled and cut into pieces, then boiled in wine, water, or vinegar to separate clean bones from the fleshy parts. This was not as might at first seem some ghastly punishment for traitors or heretics, but a high honour: the bones of the great and good were preserved as relics in this way, and could be transported to some suitable repository such as a cathedral.
Christian burial practice until about 1000 was to bury the person where they died. Mediaeval Europeans did not have the capability of fully embalming a body. Around then they began to practise the mos teutonicus, so that great rulers could be brought home even if they had died in an outlying province.
A web search gives no matches for the term. But it is featured in this week's New Scientist (issue 2320) in relation to the alleged practice of ritual dismemberment among the Mayans in human sacrifice. Many Mayan burials are missing bones. But a comparative study of Western European burials, where records are abundant and there is no suggestion of dismemberment, has shown that the mos teutonicus similarly disturbed bones, which would be jumbled up together and could easily have a few misplaced.