Why do English “pound” and German “Pfund” sound different?

The difference in pronunciation between the English word "pound" and the German word "Pfund" is primarily due to the High German consonant shift, a systematic sound change that occurred in the southern German speech area, likely during the 6th century.1

During this shift, the voiceless stop p—which was present in the early Germanic forms of both languages—underwent a transformation in the dialects that became High German. When occurring at the beginning of a word, the stop p shifted to the affricate pf, resulting in the modern German Pfund. English, which descended from North Sea Germanic, did not undergo this shift, thus retaining the original stop sound in "pound".12

Historical evidence from loanwords further confirms this timeline. Words borrowed from Latin, such as pondō, were adopted into the Germanic languages before the consonant shift occurred. Consequently, while English reflects the earlier sound in "pound," the German form "Pfund" reflects the subsequent shift of the initial p to pf.1