Tenuis

The superscript equal sign ˭ is here used to denote the Czech tenuis consonant, in this case the plosive [t˭], which lacks aspiration, in order to contrast it with its aspirated counterpart in English [tʰ].

Source: wiktionary

The tenuis becomes aspirate in Low-German.

Source: wiktionary

Since the explosive was a tenuis before a consonant we have -p m- and -t n-; these combinations were mutated to mh and nh in the following examples, the voicelessness of the tenuis being retained after its assimilation

Source: wiktionary