Simple function to add header tooltips to DT::datatable. Uses Bootstrap 3 tooltip component: https://getbootstrap.com/docs/3.3/javascript/#tooltips

dt_add_header_tooltips.R

# table_dt: DT::datatable
# tooltips: List of tooltips. Names = header names, values = tooltips.
# add_to_header_name: Will be appended to header name. Defaultly a question mark in circle.
#                     See https://getbootstrap.com/docs/3.3/components/ for more glyphicons
# tooltip_params: Parameters for Bootstrap's tooltip component.
#                 See https://getbootstrap.com/docs/3.4/javascript/#tooltips-options
#                 Will be converted to JSON with jsonlite::toJSON(tooltip_params, auto_unbox = TRUE)
#                 container = "body" is important as it always shows a tooltip on top of other elements.
dt_add_header_tooltips <- function(
  table_dt,
  tooltips,
  add_to_header_name = "<span class='glyphicon glyphicon-question-sign' aria-hidden='true'></span>",
  tooltip_params = list(
    container = "body",
    delay = list("show" = 0, "hide" = 50, placement = "auto")
  )
) {
  library(magrittr)
  library(glue)
  library(stringr)

  dt_header_names <- colnames(table_dt$x$data)

  tooltip_values <- tooltips %>%
    str_trim() %>%
    str_replace_all("\\n", " ") %>%
    str_replace_all("\\s+", " ")

  tooltip_df <- tibble::tibble(dt_header_name = dt_header_names) %>%
    dplyr::left_join(
      tibble::tibble(dt_header_name = names(tooltips), tooltip = unlist(tooltip_values)),
      by = "dt_header_name") %>%
    tidyr::replace_na(list(tooltip = ""))

  tooltips_js_list <- glue("'{tooltip_df$tooltip}'") %>%
    str_c(collapse = ", ")

  header_callback <- glue(
    "function(thead, data, start, end, display) {
    var tooltips = [{{tooltips_js_list}}];
    var tr = $(thead).find('th').each(function(i, el) {
      var tooltip = tooltips[i];
      if (tooltip != '') {
        var el = $(el);
        el.attr('data-toggle', 'tooltip');
        el.attr('title', tooltips[i]);
      }
    });

    $('th[data-toggle=\"tooltip\"]').tooltip({{jsonlite::toJSON(tooltip_params, auto_unbox = TRUE)}});
  }",
    .trim = FALSE, .open = "{{", .close = "}}"
  ) %>% as.character()

  table_dt$x$options$headerCallback <- DT::JS(header_callback)

  for (name in dt_header_names) {
    if (!is.null(tooltips[[name]])) {
      table_dt$x$container <- str_replace(
        table_dt$x$container,
        glue("(<th>)({name})(</th>)"),
        glue("\\1\\2 {add_to_header_name}\\3")
      )
    }
  }

  return(table_dt)
}

example.R

tooltips <- list(
  cyl = "Cylinders",
  mpg = "Miles per Gallon",
  am = "Transmission: 0 = Automatic, 1 = Manual"
)

table_dt <- DT::datatable(
  mtcars,
  filter = "top",
  rownames = FALSE,
  escape = FALSE,
  class = "display compact",
  # You can comment this to prevent styling of the table with Bootstrap.
  style = "bootstrap"
)

# We need to add the bootstrap dependency.
# You can specify the name of theme, see ?rmarkdown::html_dependency_bootstrap
table_dt$dependencies <- c(
  table_dt$dependencies,
  list(rmarkdown::html_dependency_bootstrap("flatly"))
)

table_dt <- dt_add_header_tooltips(table_dt, tooltips)