slender character; thin handwriting
Refers to a written character or handwriting that is narrow or thin in stroke, often used when describing someone's penmanship or printed text.
He wrote the letter in slender characters.
This book is printed in thin characters, so it's hard to read.
Compound of 細い (thin, slender) and 字 (character, letter). The reading ほそいじ is a straightforward combination of the two words.