inch
吋 is a specialized kanji for the unit of length 'inch'. It has no broader conceptual center beyond this measurement meaning.
吋 is a Japanese kokuji (国字) created by combining 口 (mouth) and 寸 (a unit of length, approximately 3.03 cm) to represent the English word 'inch'. The 口 component may suggest a borrowed word or a phonetic element, but the exact rationale is uncertain.
The left side 口 (mouth) and right side 寸 (a traditional Japanese inch) combine to form a new unit: an English inch. Think of someone mouthing 'inch' while measuring with a 寸 ruler.
For トウ, imagine measuring an inch with a toe: toe -> トウ, and the 吋 marks the spot.