Columnar transposition
The plaintext is written in rows under a keyword and read off column by column, in the alphabetical order of the keyword's letters.
The text is written row by row into a grid whose width is set by the length of the keyword. The columns are then read out in the order the keyword's letters would take in the alphabet. No letter is replaced, only moved, so the cipher is a pure transposition cipher and keeps its letter frequencies. Frequency analysis therefore gets you nowhere directly. Someone who guesses the keyword length can write the columns under each other and rearrange them until the text reads.
The method was common in military and diplomatic use well into the 20th century, often combined with a substitution step.