What is "to indorse in blank"

Indorse \In*dorse"\, v. t. [imp. & p. p. Indorsed; p. pr. & vb. n. Indorsing.] [LL. indorsare. See Endorse.] [Written also endorse.]

1. To cover the back of; to load or burden. [Obs.]

   Elephants indorsed with towers.
   --Milton.

2. To write upon the back or outside of a paper or letter, as a direction, heading, memorandum, or address.

3. (Law & Com.) To write one's name, alone or with other words, upon the back of (a paper), for the purpose of transferring it, or to secure the payment of a note, draft, or the like; to guarantee the payment, fulfillment, performance, or validity of, or to certify something upon the back of (a check, draft, writ, warrant of arrest, etc.).

4. To give one's name or support to; to sanction; to aid by approval; to approve; as, to indorse an opinion.

   To indorse in blank, to write one's name on the back of a note or bill, leaving a blank to be filled by the holder.