thru

Etymology
Modern use originated in as a phonetic and simplified spelling of  around 1839. The spelling had previously been one of several used in.

Usage notes

 * Thru is mostly used where the preposition through could be used (e.g. Monday thru Friday); it is less common as an adjective or adverb (I'm thru with the vacuuming). It is less used in formal situations, except in cases where brevity is wanted such as roadway signs or date ranges.
 * This spelling is not used in standard British English, though it can be found as an abbreviation – similar to thro' – and is recently becoming noticeable due to use by American-based companies, especially in the term drive-thru. It may also be used as an eye dialect spelling in some regions.
 * Through (or thru) is rarely used in any spelling to indicate ranges in British English; to, till, and until are used instead.
 * Thru is often used, in both American and British English, in professional drafting (5/8″ thru hole) to save space and simplify annotation on a drawing for fabrication or construction.
 * May be used by advocates of English spelling reform.