the word 'thousandth' is a transparent formation within english, combining the cardinal number 'thousand' with the ordinal suffix '-th'. the root 'thousand' originates from proto-germanic þūsundī, which likely meant a "great hundred" or a large multitude. the ordinal suffix '-th' descends from the proto-germanic -dô, -diz, used to form ordinal numbers. the logic of the construction is perfectly regular and arithmetic: the suffix '-th' attaches to the cardinal numeral to denote both position in a sequence (the one-thousandth item) and the fractional unit (one one-thousandth). this pattern is consistent with other english ordinals like 'hundredth', 'millionth', and 'tenth'.

Structure: thousand (the number 1000) + th (ordinal/fractional suffix) thousand: from old english þūsend, of germanic origin; functions as the cardinal number base. th: from old english -þa, -þe, a suffix forming ordinal numbers and fractions; of germanic origin.